Externally-strain-engineered semiconductor photonic and electronic devices and assemblies and methods of making same

ABSTRACT

Externally-strained devices such as LED and FET structures as discussed herein may have strain applied before or during their being coupled to a housing or packaging substrate. The packaging substrate may also be strained prior to receiving the structure. The strain on the devices enables modulation of light intensity, color, and electrical currents in some embodiments, and in alternate embodiments, enables a fixed strain to be induced and maintained in the structures.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/562,462 filed Sep. 28, 2017, which is a 35 U.S.C. § 371 nationalstage application of PCT/US2016/026707, filed Apr. 8, 2016 and entitled“Externally-Strain-Engineered Semiconductor Photonic and ElectronicDevices and Assemblies and Methods of Making Same,” which claimspriority to U.S. Provisional Patent Application No. 62/144,715, filedApr. 8, 2015 and entitled “Externally-Strain-Engineered SemiconductorPhotonic and Electronic Devices and Assemblies and Methods of MakingThereof”, the disclosure of each of which is hereby incorporated hereinby reference in its entirety herein for all purposes not contrary tothis disclosure.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND

Solid-state lighting technology may be employed in automobile lighting,building lighting, traffic lights, and other lighting system, andemploys semiconductor light-emitting diodes (LEDs) or otherlight-emitting diodes such as polymer or organic diodes. Solid-statelighting is distinct from other types of lighting such as fluorescentlighting that may use plasma, or other lighting that may instead employgas or filaments. Solid-state lighting emits solid-stateelectroluminescence that creates visible light without the concerns ofexcessive heat generation or parasitic energy dissipation. Beyondlighting applications, solid-state devices may be employed in powerelectronics where electrical power is controlled and converted. Highervoltage and higher power devices are necessary and may be in higherdemand in future applications, including smart grid systems as well ashybrid and electric vehicles.

BRIEF SUMMARY OF THE DISCLOSURE

In an embodiment, a mounted field-effect transistor comprising: alayered structure comprising a first layer disposed in contact with asecond layer; a channel at the interface of the first layer and thesecond layer, wherein the channel comprises a high density of electrons;at least a first and a second electrode in contact with the first layerand free of contact with the second layer, wherein the layered structureis in one of a bend-up or bend-down condition and comprises apredetermined amount of strain; and a packaging substrate, wherein thelayered structure is disposed in contact with the packaging substrateand retains at least some of the predetermined amount of strainsubsequent to disposal.

In an embodiment, a method of modulating electrical current comprising:disposing a first layer in contact with a second layer; forming achannel at the interface of the first layer and the second layer;disposing at least a first and a second electrodes in contact with thefirst layer to form a transistor; inducing strain in the transistor,wherein inducing the strain comprises applying strain to the transistorin at least one strain cycle, whereby the transistor retains apredetermined amount of strain subsequent to the at least one straincycle; disposing the transistor in a housing in a manner such that thetransistor retains the predetermined amount of strain subsequent todisposal in the housing; and subsequent to disposing the transistor inthe housing, changing an amount of strain on the channel, wherein anelectrical current in the transistor is modulated by changing the amountof the strain on the channel.

In an embodiment, a method of manufacturing a device, comprising:forming a plurality of light-emitting devices, wherein forming eachlight-emitting device of a plurality of the light-emitting devicecomprises: disposing a p-type electrode in contact with a p-typesemiconductor layer; disposing a light-active region between the p-typesemiconductor layer and at least one n-type semiconductor layer; anddisposing an n-type electrode in contact with the n-type semiconductorlayer; and applying strain in at least one strain cycle to eachlight-emitting device of the plurality of light-emitting devices,wherein each light-emitting device retains a predetermined amount ofstrain subsequent to the at least one strain cycle to produce at leastone of a bend-up or a bend-down condition in the light-emitting device.

Embodiments described herein comprise a combination of features andcharacteristics intended to address various shortcomings associated withcertain prior devices, compositions, systems, and methods. The variousfeatures and characteristics described above, as well as others, will bereadily apparent to those of ordinary skill in the art upon reading thefollowing detailed description, and by referring to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-sectional structure of an un-bent visible,infrared (IR), and ultraviolet (UV) light-emitting device (orlight-emitting diodes, LED).

FIG. 2 illustrates a cross-section of an un-bent heterostructure fieldeffect transistor (HFET or FET for field-effect transistors in short),also referred to as high-electron mobility transistors (HEMT).

FIG. 3 is an illustration of the application of strain to an LED or HFETassembly according to certain embodiments of the present disclosureprior to the assembly being packaged/disposed in the packagingsubstrate.

FIG. 4 is a graph of equilibrium electronic band diagrams of aquantum-well (QW)-region of a light-active region in an LED with variousbending conditions according to certain embodiments of the presentdisclosure.

FIG. 5 is a graph illustrating wave functions of electrons and holes inthe QW with representative bending conditions at a current density, J˜20A/cm² in an LED structure.

FIG. 6 is a graph illustrating changes in the internal quantumefficiency (IQE) and peak emission wavelength of a QW structure as afunction of curvature, as an example of IQE improvement and colortunability in the case of visible LEDs.

FIG. 7 is a schematic illustration of multi-color LEDs with the same QWactive region by modulating external strains for the generation of whitelight.

FIG. 8 shows an electronic band diagram of InAlGaN/GaN HFETs fabricatedaccording to certain embodiments of the present disclosure.

FIG. 9 shows a 2-DEG density change with various curvature changes(bending strain) for samples fabricated according to embodiments of thepresent disclosure.

FIGS. 10A and 10B illustrate two embodiments of an externally-strainedsemiconductor structure disposed on a packaging substrate.

FIGS. 11A-11C illustrate the unbent, bend-up, and bend-down conditionsof strain-effect transistor (SET) structures fabricated according toembodiments of the present disclosure.

FIGS. 12A-12B illustrate the bend-up, and bend-down conditions of HFETstructures fabricated according to embodiments of the presentdisclosure.

FIG. 13 is a diagram of exemplary embodiments of externally straineddevice types.

DETAILED DESCRIPTION OF THE DISCLOSED EXEMPLARY EMBODIMENTS

The following discussion is directed to various exemplary embodiments.However, one of ordinary skill in the art will understand that theexamples disclosed herein have broad application, and that thediscussion of any embodiment is meant only to be exemplary of thatembodiment, and not intended to suggest that the scope of thedisclosure, including the claims, is limited to that embodiment.

The drawing figures are not necessarily to scale. Certain features andcomponents herein may be shown exaggerated in scale or in somewhatschematic form and some details of conventional elements may not beshown in interest of clarity and conciseness.

In the following discussion and in the claims, the terms “including” and“comprising” are used in an open-ended fashion, and thus should beinterpreted to mean “including, but not limited to . . . .”

By this express reference, “Externally-Strain-Engineered SemiconductorPhotonic and Electronic Devices and Methods of Making Thereof,” byJae-Hyun Ryou, Shahab Shervin, and Seung Hwan Kim, is incorporated inits entirety herein; “Threshold Voltage Control of InAlN/GaNHeterostructure Field-Effect Transistors for Depletion- andEnhancement-mode Operation,” by Suk Choi, Hee Jni Kim, Zachary Lochner,Yun Zhang, Yi-Che Lee, Shyh-Chiang Shen, Jae-Hyun Ryou, and RussellDupuis, is also incorporated in its entirety by express reference;“Control of Quantum-Confined Stark Effect I InGaN-Based Quantum Wells,”by Jae-Hyun Ryou, P. Douglas Yoder, Jianping Liu, Zachary Lochner,Hyunsoo Kim, Suk Choi, Hee Jin Kim, and Russell D. Dupuis, is alsoincorporated in its entirety by express reference; and, “WaterproofAlInGaP Optoelectronics on Stretchable Substrates with Applications inBiomedicine and Science,” Rak-Hwan Kim, Dae-Hyeong Kim, Jianliang Xiao,Bong Hoon Kim, Sang-II Park, Bruce Panilaitis, Roozbeh Ghaffari₅, JiminYao₆, Ming Li, Zhuangjian Liu, Viktor Malyarchuk, Dae Gon Kim, An-PhongLe, Ralph G. Nuzzo, David L. Kaplan, Fiorenzo G. Omenetto, YonggangHuang, Zhan Kang, and John A. Rogers is also incorporated in itsentirety by express reference.

In the U.S., approximately 22% of total generated electricity isconsumed in lighting, and energy saving in lighting is expected to besignificant for green way of energy consumption. Solid-state lighting(SSL) lamps based on light-emitting diode (LED) technology are widelyacknowledged presently to be the best choice of sustainable lightingtechnology. The levels of efficiency and reliability achieved by LEDstoday are far superior to those of traditional light sources. Typicalefficacy of white LED lamp is approximately 120 lumens per watt (lm/W),which is higher than those of compact fluorescent lamps (60-80 lm/W) andincandescent sources (11-17 lm/W). The life span of a typical LED bulbis expected to be longer than 8 years. SSL lamps based on LED technologyenable very low ownership cost (offering economic benefit) and producelow carbon footprints (offering environmental benefits). Visible LEDsare conventionally used in full-color display, automotive lighting,traffic light, liquid-crystal display (LCD) back-light unit, and othersolid state lighting applications.

Discussed herein are systems and methods aimed at improving performancecharacteristics and functionalities of photonic and electronic devices.These devices comprise semiconductor materials with polarizationproperties and are fabricated to address technical challenges innext-generation solid-state lighting, electronics, electro-mechanics,and photonics. In an embodiment, electronic band structures of polarsemiconductors have a layer that consists of anions or cations on asurface such as GaN-based (Al_(x)Ga_(y)In_(1−x−y)N) and ZnO-based(Mg_(x)Zn_(y)Cd_(1−x−y)O) materials, this layer may be engineered formultifunctional and/or high-performance devices by external strains. Asused herein, “external strain” means a strain applied to a devicestructure such as an LED or FET that induces a predetermined amount ofstrain (first state) and leaves the structure in a bent condition(second state), similar to what is discussed in FIGS. 11B, 11C, 12A, and12B, below in contrast to the un-bent structures in at least FIGS. 1, 2,and 11A, which are illustrated for reference to show the components ofthe respective structures. Conventionally, visible, infrared (IR), andultraviolet (UV) LEDs and FETs may be flat structures (as indicated inFIGS. 1, 2, and 11A).

Devices fabricated according to embodiments of the present disclosurehave retained a predetermined amount of external strain that is appliedprior to and/or during mounting the device(s) to a packaging substrate.At least two types of devices are discussed herein, light-emittingdevices (LEDs) and field-effect transistors (FETs) that may be ofvarying types including heterostructure field-effect transistors (HFET).An LED fabricated according to embodiments of the present disclosure maycomprise a high internal quantum efficiency (IQE) due to fixed eternalstrain, or may comprise external strain that may be varied in order tochange the wavelengths (color) of the light generated by the LED(s). Thewavelength (color) of the LED(s) includes wavelengths in the IR and UVranges as well as within the visible color range.

An HFET may comprise a source, drain, and gate as discussed below andmay comprise an external strain, FETs with source and drains but with nogate may be modulated by variable external strain and may be referred toas strain-effect transistors. An FET that has had external strainapplied and has no gate may be referred to as a strain-effect transistor(“SET”).

The technologies disclosed herein address critical issues in solid-statelighting applications, smart-grid system, and power electronics.Specifically, concepts introduced herein include visible, IR, and UVLEDs having high quantum efficiencies (QE) in wide-range of operatingcurrents with color-changing capability, as well as, transistors thatcan be controlled by external strain, that is, strain-effect transistors(SETs). The effects of polarizations in III-N materials have bothtechnical challenges and benefits in the devices. These effects alsoplay a role similar to other materials having polarization propertiesincluding II-VI-based semiconductors, for example, ZnO. Therefore, thefollowing discussion on the operating principle focuses on III-Nmaterials as an exemplary embodiment, but the concepts and principlesdisclosed herein are not limited to III-N materials.

Strained heteroepitaxial growth works well only for the growth along ac-direction of the wurtzite structure on (0001) planes. Hence, mostIII-N devices are grown epitaxially in a polar direction with aGroup-III-element-terminated surface such as gallium (Ga)-polar. Suchmaterial structures contain spontaneous and piezoelectric polarizations,which induce charges at the interfaces and resulting gradient inelectronic band structure of the layers. This is commonly called bandbending, a term conventionally used to describe local changes in energyoffset near the junction of a semiconductor's band structure, whichresults in bending observed in band diagrams which are plots of energyvs. distance. Conventionally, “band bending” does not refer to thephysical bending of a structure such as the method used to fabricate anentire externally strained semiconductor structures described herein. Toavoid the confusion between electronic band bending and semiconductorstructure bending by applying external strain, the term “band tilt” maybe used herein to refer to the electronic band bending and the “bending”the occurs in response to the application of external strain is abending (displacement from an initial flat position) of a semiconductorstructure. The band tilt results in quantum-confined Stark effect (QCSE)in InGaN/GaN quantum wells (QW) of LEDs and 2-DEG in InAlGaN/GaN HFETs.The QCSE in QWs is one of the technical challenges to be overcome. It isrelated to a reduced radiative recombination rates and also possibly oneof the origins of efficiency droop in visible LEDs. In an attempt toavoid or mitigate the QCSE, epitaxial growth in non-polar and semi-polardirections has been marginally successful only on native substrates.This approach may not be easily implemented in real-life devices due tovery expensive and low-throughput nature of the native substrates. Theband tilt in HEMTs has provided the benefit in depletion-mode(normally-on) transistors. However, band tilting poses technicalchallenges for the development of enhancement-mode (normally-off)transistors that are preferred in power-switching applications. Theengineering of polarization has been focused mainly on the effects fromdifferential spontaneous polarization and lattice strain set bydifference in layers of epitaxial structures. There was not a greatdegree of freedom in the engineering of polarization, unless anepitaxial structure was modified. In this disclosure, the polarizationeffects beyond the limit of lattice strain created by applying externalstrain in flexible III-N structures are discussed. Theseexternally-strained structures may be employed in various devices formultiple functionalities and performance improvements.

Light-Emitting Diodes (LEDs)

In an embodiment, external strain may be applied to one or more LEDssimultaneously. The LED may have a high internal quantum efficiency(IQE) subsequent to the application of strain. This application ofstrain may occur before, during, or both before and duringcoupling/mounting the LED in a packaging component. The LED may retain apredetermined amount of the applied strain, which may be applied in oneor more strain cycles as discussed herein. This retained strain may bereferred to as a “fixed external strain” or “fixed strain.”

In another embodiment, LEDs may comprise red, green, blue (RGB) colors,and the external strain that is applied to and retained by the LED maybe selected in part on the color of the LED. These LEDs that compriseRGB colors may be used in various combinations to produce white LEDlight. In alternate embodiments, LEDs comprising external strain maycomprise variable external strain and may be modulated (controlled,color-changed) using this variable external strain. These devices thathave external strain that may or may not be variable external strain maycollectively be referred to as “externally strained devices,”“externally strained structures,” or “externally strained components.”In some embodiments, the substrate may be fabricated in order to induceand/or maintain an optimum amount of strain on the LED or otherstructure such as an FET as discussed herein, if the strain is variablein a strained FET, it may be referred to as an SET. In some embodiments,the LED structure is pre-strained prior to assembly with the packagingsubstrate, and in alternate embodiments the LED or FET structure isstrained when coupled to or otherwise disposed on the packagingsubstrate. In other embodiments, the LED or FET structure may bestrained prior to and during the disposal/coupling with the packagingsubstrate

Field-Effect Transistors (FETs) and Strain-Effect Transistors (SET)

In an embodiment, a field-effect transistor (FET) may also have externalstrain applied and may retain at least some of the applied strain, whichmay be applied in one or more cycles. In an embodiment, the FET maycomprise a source electrode (source), a drain electrode (drain), and agate electrode (gate). In this embodiment, external strain applied tothe FET may create a predetermined strain that may be a fixed strain. Insome embodiments, FETs may comprise a variable external strain, incontrast to a fixed strain, that may be used to enable modulation ofelectrical currents using strain applied to a transistor's channel,instead of a gate. These structures may be referred to herein as“strain-effect transistors” (SETs).

In particular, discussed herein are (1) optical output power andefficiency of visible, IR, and UV light-emitting diodes (LEDs) areimproved by optimum bending of semiconductor structures; (2) emissionwavelength of LED which are changed by external strain which is relatedto wavelength-tunable LEDs; (3) red, green, blue (R/G/B) LEDs and otherLEDs with different colors are realized from the same materials bycorrugated and other-form semiconductor structures for phosphor-freewhite LEDs; (4) conductivity in a channel of transistors is modulated bybending of semiconductor structures; (5) a new device named astrain-effect transistor (SET). The new concepts and processes areexpected to provide a solution that can overcome certain technologicallimitations in current energy-saving devices and systems.

Semiconductor materials are a building block of photonics andelectronics that enable current information technology and havepotential for next-generation green sustainable technology. Mostsemiconductor-based photonics and electronics, however, currently faceboth technical and economic challenges to be further competitive forcurrent and next-generation applications.

The semiconductor devices discussed herein are fabricated using methodsaimed to improve performance, affordability, and functionality of thesedevices, for example, for devices such as the types of semiconductors tobe used in green and information systems. This disclosure discussesseveral examples of technologies where the concept of this disclosurecan be applied.

Referring to the cross-sectional structure 100 of FIG. 1, an un-bent LEDstructure 100 is illustrated for reference. The structure 100 comprisesa light-generation active region 114 comprising a plurality of pairs oflayers that each comprise a quantum well (QW) (108) and quantum-wellbarrier (QWB) (102). The light-active region 114 is sandwiched betweentwo regions of p-type layers (112) and n-type layers (104) withpositively- and negatively-charged conducting properties, respectively.The devices also include a p-type electrode (110) and an n-typeelectrode (106). It is appreciated that the thicknesses and relativethicknesses of the attributes of the LED structure 100 are merelyillustrative and may not represent thicknesses of each of the discussedlayers.

By employing various embodiments of the present disclosure, theefficiencies of light-emitting devices including LEDs and laser diodescan be improved, including a new method of tuning colors of theselight-emitting devices. In an embodiment, an LED device according toembodiments of the present disclosure may comprise the same componentsas FIG. 1 but may be in a bend-up or a bend-down strain condition asillustrated in FIGS. 11B, 11C, 12A, and 12B, and may be configured tocomprise a fixed strain or a variable strain. As used herein, the term“bend up” condition refers to a device that is concave relative to itsangle to a packaging substrate (e.g., FIGS. 11B and 12A), and the term“bend-down” refers to a device that is convex relative to a packagingsubstrate (e.g., FIGS. 11C and 12B).

Semiconductor devices are also used in power switching and conversionapplications in the field of ‘power’ electronics. Silicon (Si)semiconductors, despite inefficiencies in conversion and switching ofthe energies, have been a dominant material in such applications. Newapplications where Si semiconductors are desirable to use may employsignificantly higher current and voltage handling capacities thanconventional applications. As discussed herein, devices based on GaN andother materials discussed herein may be more energy efficientalternative to current devices using Si. Among the many fundamentalproperties of GaN and related materials, a high breakdown field and ahigh saturation velocity can be used for high voltage and high currentapplications, respectively. GaN-based transistors and diodes maytherefore be desirable components in many systems used for powerswitching and conversion.

In some embodiments, for power applications, it is desirable that thedevice is turned off when electrical power is not being applied to thedevice. In other words, for power switching applications, the “on/off”or power switch needs to be set to “off,” without power, for the saferoperation of switches. This is called a “normally-off” operation and isbetter for “fail-safe” mode operation when the main current flowingthrough the switch is to be stopped. When a device is in an a fail-safeoperation mode, the control electronics do not function properly withoutpower.

FIG. 2 illustrates a cross-section of an un-bent transistor 200 that maybe strained according to certain embodiments of the present disclosure.The transistor shown as the cross-section 200 may comprise GaN materialsor other materials as discussed such as heterostructure field effecttransistors (HFETs), also known as high-electron mobility transistors(HEMT). The HFETs may have a cross-sectional structure 200 as discussedbelow and are not in a strained bend-up or bend-down condition. In anembodiment, a first layer 208 comprises In_(x)Al_(y)Ga_(1−x−y)Nmaterials and a second layer 202 comprises GaN. In alternateembodiments, the first layer 208 comprises In_(x)Al_(y)Ga_(1−x−y)N,where x may range from 0 to about 1, and y may be from 0 to 1. Thesecond layer 202 comprises Mg_(x)Zn_(y)Cd_(1−x−y)O, where x may rangefrom 0 to about 1, and y may be from 0 to 1. In alternate embodiments,the second layer 202 comprises In_(x)Al_(y)Ga_(1−x−y)N and the firstlayer 208 comprises Mg_(x)Zn_(y)Cd_(1−x−y)O. In some embodiments x+y<1,and either x or y may be equal to 0.

Near the interface of the first and the second layers 208 and 202, ahigh concentration (or density) of electrons is formed as a channel 210.This channel 210 may be typically referred to as 2-dimensional electrongas (2-DEG). Due to formation of the 2-DEG, the channel 210 for currentflow exists between a source electrode 204 and a drain electrode 206without applying bias at a gate 212. The channel 210 is a region formedby the interface of layers 202 and 208 and is indicated in FIG. 2 withshading for illustrative purposes. A “normally-on” operation may bedescribed as when the channel 210 can be off only when a gate 212 hasapplied negative biases. There are different ways to achieve“normally-off” operation in GaN and MgZnCdO-based HFETs. Discussedherein are embodiments of a method to achieve normally-off conditionsand to control the currents between a source 204 and a drain 206 in animproved LED structure.

The systems and methods discussed herein may provide (1) improvedefficiency of light-emitting devices beyond the limit of currenttechnology platform having fixed internal strain; (2) Color changingfunctionality of light-emitting devices consisting of the samematerials; (3) Normally-off (enhancement mode) transistors to be used inpower electronics for fail-safe operations; and (4) Newelectro-mechanical devices comprising a channel controlled by externalstrain applied to the FET structure either before or after assembly, incontrast with the use gate bias to control the channel as in the case oftraditional FETs.

Design and Process

In an embodiment, flexible devices were fabricated using inorganicsemiconductors. Subsequently, external strain was applied tosemiconductor structures previously grown on non-flexible single-crystalsubstrates. Historically, applying strain to semiconductor waschallenging due to the brittle nature of the semiconductor materials andtheir substrates. The bend-up and bend-down conditions discussed hereinwere achieved when the strain was applied externally to the entiresemiconductor structure, so the application of strain must be done in away as to not compromise the components of the structure, including thesometimes brittle substrate. As shown and discussed herein, whenepitaxial structures become thin films (i.e., films comprising athickness less than about 5 μm) and are fabricated without using rigidsubstrates, the semiconductor layers on flexible substrates canwithstand a high degree of strain, at least some of which is retained inthe structures, without comprising the integrity of the structures.

The external strain applied herein was applied to two different types ofstructures, light-emitting diodes (LEDs) and heterostructurefield-effect transistors (HFETs), discussed above, which may becollectively referred to as FETs since the external strain may beapplied in other embodiments to other types of FET devices.

FIG. 13 is a diagram 1300 of exemplary embodiments of externallystrained device types. FIG. 13 is provided as a high-level overview ofthe devices discussed herein, and illustrates that there may be aplurality of LEDs at block 1302, and when an external strain is appliedas discussed above to create a bend-up or a bend-down structure, aplurality of LEDs may be formed at block 1308 that have a high IQE and afixed external strain, and a plurality of LEDs may be formed at block1310 that comprise RGB colors with different fixed external strains, andsome at block 1314 that are white (not RGB) LED with external strains.In an embodiment, there may also be a plurality of LED formed at block1312 that can have their colors changed by a variable external strain asdiscussed above. At block 1306, there may be a plurality of HFETs, andwhen the external strain is applied by bending the FET structures atblock 1304, a plurality of FETs comprising source drains and gates areformed at block 1316 with a fixed external strain, and a plurality ofSETs comprising a source and a drain but no gate, and comprisingvariable external strain.

Referring now to FIG. 3, conventionally, flexible semiconductorstructures were focused on flexibility and stretchability of devices tobe employed in curved, foldable, or rollable applications. FIG. 3illustrates the flexible properties of an LED or FET, i.e., the abilityof the LED or FET device to be externally strained is leveraged toimprove and extend device functionality according to the embodimentsdiscussed herein. The devices manufactured according to embodiments ofthe present disclosure are in contrast to those flexible semiconductorsfabricated for use in applications that required the flexibility of thepackaged/mounted device itself. However, in some applications,externally strained devices may be introduced to applications where thepackaged/mounted device is flexible, but the devices may be externallystrained prior to disposal in the packaging. In schematic 300, strain isapplied to an LED or FET or other assembly as discussed herein, thestrain may be applied at two or more points from two directions that maycomprise mirrored angles 302 and 304, or may have strain applied inother directions 306, 308, 310, such strain may be uniaxial, biaxial, ormulti-axial, and may generate a shape that is concave (as illustrated)or convex (not pictured) with respect to a normal plane 312.

Applying uniaxial or biaxial strain (stretching) on the flexiblesemiconductor structures may be challenging due to the complexity of FETand LED structures. In an embodiment, bending was utilized to applyexternal strain, for modification of performance and functionality. Thebending strain can be applied prior to packaging/seating the devices,but may also be easily applied in real-world devices during packaging.As used herein, the term “packaging” may mean the assembly of an LED,FET, or other such semiconductor structure on to a substrate, that is,the coupling of a first structure to a second structure.

Light-emitting devices: In light-emitting devices, QCSE may present achallenge. The total polarization of a QW by external strain is:P _(ex) =P _(sp) ,QW−P _(sp) ,QWB+P _(pz) ,QW(_(ϵ1))−P _(pz),QWB(_(ϵ2))  (1a)P _(ex) =P _(total) ,QW−P _(total) ,QWB=ΔP _(total)  (1b)where P_(sp),QW and P_(sp),QWB are spontaneous polarization fields of QWand QWB, respectively; P_(pz),QW is a piezoelectric polarization;P_(pz),QWB is a piezoelectric polarization field of QWB; and _(ϵ1) and_(ϵ2) strains are from combinations of lattice mismatch and externalbending in QW and QWB, respectively.

FIG. 4 illustrates the QCSE for LEDs in three bending conditions: thathave not been bent, that have been bent up (“bend-up condition”) andthat have been bent down (bend-down condition”). The condition ofbend-up and bend-down can be changed depending on the location of QWsand location of a neutral plane of the whole structure where net strainis zero during bending. External bending strain changes the QCSE, asillustrated in FIG. 4. The results for the no-bend condition (i.e., nostrain has been applied to the LED) indicates the significant QCSEevidenced from band tilt in the QW region. Conduction and valence bandsbecome more flattened and more tilted for bend-up and bend-downconditions, respectively. The total polarization difference between QWand QWB in Eq. 1a and 1b is used to address the induced band tilt andresulting QCSE. In particular, for the bend-up conditions, the QCSEbecomes mitigated. When charged carriers are injected, spatialdistribution of electrons and holes and effective bandgap energies aredependent on the band tilt in the QW.

FIG. 5 illustrates wave functions of electrons and holes. In the bend-upcondition where strain was applied to the semiconductor structure andsubstrate, the wave functions of the electrons and the wave functions ofthe holes move closer to each other and are more overlapping than thosestructures fabricated in the bend-down condition. In contrast, the wavefunctions and holes are farther separated with increasing strain appliedfrom a no-bend condition to the bend-down condition. The degree ofseparation in the wave functions affects the oscillator strength ofcarriers; hence, a radiative recombination probability and a radiativerecombination rate. A radiative recombination rate competing againstother recombination rates, i.e., rates of different non-radiativerecombinations including Shockley-Read-Hall and Auger recombinations,determines internal quantum efficiency (IQE) of the light emitters. TheIQE was improved by increasing a radiative recombination rate whileminimizing non-radiative recombination rates. The modification in QCSEchanges dominant transition energies, and therefore also the lightemission wavelengths and colors.

Referring now to FIG. 6, by applying external compressive strain(bend-up conditions), the IQE improves comparing to a no-bendingcondition and it keeps increasing with increasing curvature. Also,mitigated QCSE by external compressive strain makes effective bandgapwider. When bending condition changes from no bend to bend-up condition,emission peak from the QW shifts toward shorter wavelength (blue shift).On the other hand, bending down induces red-shift (peak wavelengthtoward longer wavelength). Computational demonstration on the effect ofexternal strain shows that bending the LED by external loading canimprove performance and change the color of light emitters. This findingis utilized in further enhancement of IQE of LED-based SSL lamps.

FIG. 7 is a schematic illustration of multi-color LEDs with the same QWactive region by modulating external strains. This packaging substrateis different from the substrate used for deposition of semiconductormaterials and structures because the packaging substrate is not flat,rather, it comprises a curve that matches the strain induced in an LEDby bending (application of external strain). By changing direction anddegree of external bending strain, the color of flexible LEDs can betuned. This color changing characteristics can also be utilized in whiteLED lamps by combining red (R), green (G), and blue (B) colors on thesame chip that can replace phosphor-converted LEDs (PC-LEDs) used inwhite SSL lamps. RGB-LED-combination LEDs have the potential for higherefficiency, while being less expensive than PC-LEDs. Higher cost isrelated to integration of multiple LED chips having different colors.Described herein is the concept of employing different color LEDs on asame chip using the same LED structure, which is expected to lower themanufacturing cost.

Field-effect transistors and strain-effect transistors (SETs): Turningback to FIG. 2, III-N-based HFETs have a channel with a 2-DEG (210).This 2-DEG 210 is formed by polarization effects and the concentrationand mobility of the electrons in 2-DEG 210 governs transfercharacteristics of the transistors. For an FET, the channel may becontrolled by an electric field through gate bias, and for an SET, thechannel is controlled by strain without bias on the gate.

Referring now to FIG. 2 and FIG. 8, the electronic band diagram ofInAlGaN/GaN HFETs comprises an InAlGaN Schottky barrier layer (indicatedby 801, and corresponding to the first layer 208) and a GaN layer (802,corresponding to the second layer 202).

The heterostructure inherently forms a triangular QW at the interfaceand 2-DEG (indicted by 803, corresponding to the 2-DEG 210) is formedwhen the depth of the QW is lower than the Fermi level 804. Theconcentration of 2-DEG 803 increases with increasing depth of QW belowFermi level. In the case of HFETs on nonflexible substrates, the 2-DEG803 density (concentration) is fixed once the epitaxial layer structureis designed. Transfer characteristics of the channel are controlled bythe field applied on the gate in traditional (conventional) FETs. Asdiscussed herein, the 2-DEG 803 density can be further engineered byapplying external strain to an FET as opposed to using the gate.

Referring now to FIG. 9, for an In_(0.32)Al_(0.72)N/GaN normally-offHFET, a channel starts to form with a higher density of 2-DEG withincreasing curvature of devices strained to bend-up conditions(indicated by the open circles). The range of modulation of 2-DEGdensity with curvature was highest in the case of anIn_(0.2)Al_(0.8)N/GaN normally-on HFET (indicated by the open and closedtriangles), while that of an In_(0.2)Ga_(0.8)N/Al_(0.15)Ga_(0.85)N/GaNnormally-off HFET is negligible (indicated by the open and closedinverse triangles). The effects of external strain on 2-DEG may be due Ipart to a degree and/or direction of bending/external strain applied,which may collectively be referred to as a “bending status,” and theSchottky barrier, which is the potential energy barrier for electronssuitable for use as a diode and formed at a metal-semiconductorinterface.

Threshold voltage, V_(th), of the III-N HFETs can be expressed as:V _(th)=ϕ_(B) /e−dσ/ε−ΔE _(C) /e+E _(F0) /e  (2)where ϕ_(B) is a metal-semiconductor Schottky barrier height; σ a chargeinduced at the interface by polarizations, d thickness of the InAlGaNlayer, ΔE_(C) a conduction-band offset, E_(F0) an electron energydifference between Fermi level and the conduction band edge of the GaNlayer, e the elementary electron charge, and ε a dielectric constant ofInAlGaN. When V_(th) is negative value, the transistor is operated in adepletion (normally-on) mode. When V_(th) is positive value, thetransistor is operated in an enhancement (normally-off) mode. Bymodifying strain, charge (σ) is modified to engineer threshold voltageof devices, as described in Eq. 2. In an embodiment, a 2-DEG channel canbe controlled by bending strain using an optimized structure and withoutemploying a gate bias. This effect may be employed to change theoperation mode of HFET between the normally-off mode and the normally-onmode. Actively controlled bending can also be engineered to realizestrain-effect transistors (SETs).

Turning now to FIG. 10A, the schematic cross-section 1000A ofpackaged/mounted device comprises a structure 1006 which may be an FET,SET or an LED structure in a bend-up condition and disposed on andbonded to a packaging substrate 1002 with a curved surface having anoptimized bending strain. In an embodiment, the “optimized bendingstrain” is defined as a minimum strain and/or a range of bending straininduced in the structure 1006 that is required to generate a desiredeffect for an end application. This bending strain may be induced usingone or more strain cycles, which may be of similar or varying loads andtimes. In various embodiments, the integration of LED or FET structures1006 and packaging substrates 1002 may be described as mounting ordisposal and may be achieved by chip bonding and separation of asubstrate that was used for deposition of structure.

As used herein, the term “strain cycle” may be employed to describe whenbiaxial, uniaxial, or combinations of both strains are applied to theFET structure for a predetermined period of time and removed from theLED or FET structure 1006 or 1008 (FIG. 10B below) in whole or in partafter the predetermined period of time. Strain may be applied to an FETstructure in a single strain cycle or in multiple strain cycles, and itmay be applied to the FET structure in a contiguous process of assemblyinto a packaging structure 1002, or may be pre-strained and then laterassembled. The packaging substrates may also be strained in one or morecycles, and the assembly process of the semiconductor structure 1006 (or1008) to the packaging substrate 1002 (or 1004) may in some embodimentsinduce additional strain in one or both components.

In an embodiment where a plurality of strain cycles are employed, eachstrain cycle may use the same or different amounts of strain, may beapplied in the same or different directions or in multiple directionssimultaneously or near-simultaneously, and may be employed for varyingamounts of time depending upon the strain desired for the endapplication and the composition of the LED or FET structure 1006, 1008(FIG. 10B), or others.

In an embodiment, the bend (curve) in FIG. 10A of the packagingstructure 1002 is concave, and the LED or FET 1006 may be pre-strainedprior to being disposed/seated/coupled to the packaging structure 1002.This pre-strain may be applied to bend the semiconductor structure 1006to fit into the packaging structure 1002, where the strain will bemaintained. In an alternate embodiment, the pre-strain may be appliedprior to disposal in part, with the remainder of the strain beingapplied and maintained after disposal of the LED or FET semiconductorstructure 1006 on the packaging structure 1002.

In an alternate embodiment in FIG. 10B, the structure 1000B comprises anLED or an FET structure (1008) and on a packaging substrate (1004) witha curved surface having an optimized bending strain. The bend in FIG.10B of the packaging structure 1004 is convex, and the LED or FET 1108may be pre-strained in prior to being disposed/seated/coupled to thepackaging structure 1004. This pre-strain may be applied to bend the LEDor FET structure 1008 to match the strain (bend/curvature) of thepackaging structure 1004, where the strain will be maintained once theFET structure 1008 is disposed. In an alternate embodiment, thepre-strain may be applied prior to disposal in part, with the remainderof the strain being applied and maintained after disposal of the FET1008 on the packaging structure 1004.

Turning to FIG. 11A, in an embodiment, the strain-effect transistor(SET) 1100A is first shown in an unbent condition to illustrate therelative position of components, and comprises a first layer 1106, whichmay comprise In_(x)Al_(y)Ga_(1−x−y)N materials and a second layer 1102which may comprise GaN. It is appreciated that the SET 1100 isillustrated in FIG. 11A in an un-strained condition of an SET prior tobending depending upon the embodiment. The SET 1100 may be configured,due to the absence of a gate similar to the gate 212 in FIG. 2, to be afixed strain SET. FIGS. 11B and 11C illustrate the bend-up (FIG. 11B)and bend-down (FIG. 11C) conditions for SET 1100B and SET 1100C,respectively. SET 1100B and SET 1100C which are variable-strain SETs,the features are common to the SETs in all bending conditions and arediscussed below. In FIG. 11B, near the interface of the first layer 1106and the second layer 1102, high concentration of electrons is formed asa channel (1108). The structure also contains source (1104) and drain(1110) electrode. The structure is similar to HFET structure of FIG. 2;however, it does not have the gate (212). Instead of using a gate (212),the channel is modulated by external strain, not by gate voltage inSETs. In an embodiment, the SET 1100A may be strained to a bend-up 1100Bor a bend-down 1100C condition using one cycle of strain applied for apredetermined time. In an alternate embodiment, multiple strain cyclesmay be applied to the SET 1100A, these strain cycles may be for thesame, similar, or differing amounts of strain and may be applied fordifferent periods of time, depending upon the composition of the SET1100A and the bend-up or bend-down condition of the substrate orpackaging structure or the degree of strain appropriate for the endapplication.

In an embodiment, as illustrated in FIGS. 12A and 12B, an HFET (FET)structure may comprise the gate 212 in a strained condition similar tothe bend-up or bend-down conditions 1200A and 1200B, this strainedcondition may be which may be referred to as a fixed strain or a fixedexternal strain, and an LED structure may also comprise a similar fixedstrain in a bend-up or bend-down condition. The externally straineddevice embodiments are illustrated in FIGS. 12A and 12B, where the FET(or SET) cross sections 1200A and 1200B illustrate FET (or SETs) in thebend-up (1200A) or bend-down (1200B) conditions with a gate 1212.Therefore, these embodiments are fixed strain embodiments. In alternateembodiments, the FET and LED structures discussed herein may notcomprise this gate 212 and may instead comprise strain that may bereferred to as “variable strain” or “variable external strain.”“Variable strain” is the term used to describe a strained device thatcomprises an amount of strain induced prior to and/or during couplingwith the substrate that can be varied subsequent to coupling. Thevariable strain may be employed to use the channel 1108 to modulate thecurrent.

In an embodiment, a single SET, FET, or LED may be disposed in a housingto compose a device used for light generation and/or current modulation.In an alternate embodiment, a plurality of SET, FET, or LEDs may bedisposed simultaneously or in sets of at least two at a time on apackaging substrate comprising a plurality of housings for one or moredevices used for light generation and/or modulation.

Exemplary embodiments are disclosed and variations, combinations, and/ormodifications of the embodiment(s) and/or features of the embodiment(s)made by a person having ordinary skill in the art are within the scopeof the disclosure. Alternative embodiments that result from combining,integrating, and/or omitting features of the embodiment(s) are alsowithin the scope of the disclosure. Where numerical ranges orlimitations are expressly stated, such express ranges or limitationsshould be understood to include iterative ranges or limitations of likemagnitude falling within the expressly stated ranges or limitations(e.g., from about 1 to about 10 includes, 2, 3, 4, etc.; greater than0.10 includes 0.11, 0.12, 0.13, etc.). For example, whenever a numericalrange with a lower limit, R_(l), and an upper limit, R_(u), isdisclosed, any number falling within the range is specificallydisclosed. In particular, the following numbers within the range arespecifically disclosed: R=R_(l)+k*(R_(u)−R_(l)), wherein k is a variableranging from 1 percent to 100 percent with a 1 percent increment, i.e.,k is 1 percent, 2 percent, 3 percent, 4 percent, 5 percent, . . . , 50percent, 51 percent, 52 percent, . . . , 95 percent, 96 percent, 97percent, 98 percent, 99 percent, or 100 percent. Moreover, any numericalrange defined by two R numbers as defined in the above is alsospecifically disclosed. Use of broader terms such as “comprises,”“includes,” and “having” should be understood to provide support fornarrower terms such as “consisting of,” “consisting essentially of,” and“comprised substantially of.”

While exemplary embodiments of the invention have been shown anddescribed, modifications thereof can be made by one skilled in the artwithout departing from the scope or teachings herein. The embodimentsdescribed herein are exemplary only and are not limiting. Manyvariations and modifications of the compositions, systems, apparatus,and processes described herein are possible and are within the scope ofthe invention. Accordingly, the scope of protection is not limited tothe embodiments described herein, but is only limited by the claims thatfollow, the scope of which shall include all equivalents of the subjectmatter of the claims. Unless expressly stated otherwise, the steps in amethod claim may be performed in any order and with any suitablecombination of materials and processing conditions.

What is claimed is:
 1. A method of manufacturing a device, comprising:forming a plurality of light-emitting devices, wherein forming eachlight-emitting device of the plurality of light-emitting devicecomprises: disposing a p-type electrode in contact with a p-typesemiconductor layer; disposing a light-active region between the p-typesemiconductor layer and at least one n-type semiconductor layer;disposing an n-type electrode in contact with the n-type semiconductorlayer; and producing a bend-up or bend-down strain condition by applyingstrain in at least one strain cycle to the each of the light-emittingdevices of the plurality of light-emitting devices, wherein the eachlight-emitting device retains a predetermined amount of strainsubsequent to the at least one strain cycle to produce a strainedlight-emitting device; and disposed each strained light-emitting deviceof the plurality of light-emitting devices in a packaging structure,wherein the packaging structure has a matching curvature to that of thestrained light-emitting device.
 2. The method of claim 1, wherein eachstrained light-emitting device retains the predetermined amount ofstrain subsequent to disposal in the packaging structure.
 3. The methodof claim 2 further comprising inducing additional strain on at leastsome of the plurality of strained light-emitting devices during thedisposal of each strained light-emitting device of the plurality oflight-emitting devices into the packaging structure, wherein inducingthe additional strain comprises applying strain to the at least some ofthe plurality of light-emitting devices in at least one additionalstrain cycle.
 4. The method of claim 1, wherein the light active regioncomprises a plurality of quantum wells and a plurality of quantum wellbarriers, wherein each quantum well of the plurality of quantum wells isdisposed in an alternating fashion with each quantum well barrier of theplurality of barriers.
 5. The method of claim 1, wherein color, emissionwavelength, efficiency, or a combination thereof in the pluralitylight-emitting devices is modulated by adjusting a variable strain onthe light-active region.
 6. The method of claim 1, wherein the device isa visible, infrared, or ultraviolet light generating device.
 7. Themethod of claim 1, wherein the matching curvature is concave.
 8. Themethod of claim 1, wherein the matching curvature is convex.
 9. Themethod of claim 1, wherein the strain is at least one of uniaxial andbiaxial.
 10. The method of claim 1 further comprising straining thepackaging structure prior to disposing the each strained light-emittingdevice in the packaging structure to achieve a predetermined packagingstructure strain.
 11. The method of claim 1, wherein disposing eachstrained light-emitting device in the packaging structure comprisesapplying additional strain in at least one direction to at least one ofthe packaging structure and the strained light-emitting device.